Front-projection-type image display apparatuses are gradually spreading due to widespread personal computers and digital television broadcasting.
A typical example of the front-projection-type image display apparatus is a three-panel color projector employing three spatial light modulators made of, for example, liquid crystal display elements. The three-panel color projector employs a strong light source such as a metal halide lamp to emit a white beam. The white beam is separated into three primary-color beams, which are guided to the spatial light modulators, respectively. The spatial light modulators modulate the three primary-color beams according to image signals. The modulated beams are combined into a combined beam, which is projected from a projection lens to form an image on a screen.
The image display apparatus of this type has an illuminating optical system that guides a beam from the light source and illuminates the spatial light modulators with the beam. The illuminating optical system must efficiently and uniformly illuminate the spatial light modulators with a beam of finite size emitted from the light source.
An example of the illuminating optical system is an integrator optical system. Examples of the integrator optical system include a fly-eye lens array optical system and a rod integrator optical system.
The fly-eye lens array optical system employs a fly-eye lens array that consists of a plurality of small-diameter lenses arranged in a fly-eye configuration (two-dimensional configuration), the shape of each lens being similar to the shape of an illuminated area. The fly-eye lens array receives light flux and divides the light flux into beams lens by lens. The beams are combined together to provide an efficient uniform illumination beam. This beam forms a light source image at the pupil of a projection lens.
The rod integrator optical system employs a rod integrator having an entrance face and an exit face. The shape of each of the entrance and exit faces is similar to the shape of an illuminated area. Light flux from a light source is condensed and is efficiently made incident to the entrance face of the rod integrator. The incident light flux is reflected multiple times in the rod integrator and is emitted from the exit face of the rod integrator. The emitted beam from the rod integrator is uniform over the exit face of the rod integrator and is focused under a given magnification on an illuminated area (spatial light modulator) to thereby illuminate the illuminated area efficiently and uniformly. The rod integrator optical system forms a plurality of light source images, whose number is equal to the number of reflections in the glass rod, at the pupil of a projection lens, the shape of each light source image corresponding to the shape of an opening of the glass rod.
In this way, the flay-eye lens array optical system and rod integrator optical system form each a light source image at the pupil of a projection lens.
The projection lens of the front-projection-type image display apparatus, in particular, that of the three-panel color projector must have a sufficiently long back focus and a telecentric characteristic to allow a color combining unit such as a prism to be installed on the light source side of the projection lens.
The projection lens may be a zoom lens having a variable focal length. To make the zoom projection lens compact, the lens must be configured such that a pupil position is variable according to a focal length varying operation (zooming operation). Varying the focal length of the zoom projection lens with the size of an aperture (diaphragm) of the lens being fixed results in varying the F-number of the lens.
According to the image display apparatus mentioned above, the shape of the opening of the projection lens is circular. In the illuminating optical systems mentioned above, the shape of a light source image formed at the pupil of a projection lens is not always circular. For example, the fly-eye lens array optical system forms a rectangular integrator image at the pupil of a projection lens, and the rod integrator optical system forms a rectangular rod opening image at the pupil of a projection lens.
Accordingly, the projection lens having a circular opening will have extra open sections around the light source image, to pass unwanted light through the extra open sections to deteriorate the quality of displayed images.
To avoid this problem, some related arts (Japanese Patent Application Laid-open Nos. 5-303085 and 9-274253) arrange a rectangular diaphragm in the vicinity of a projection lens and another related art (Japanese Patent Application Laid-open No. 11-281923) arranges a light shield having a rectangular aperture in the vicinity of a fly-eye lens array that is arranged on the light source side of a spatial light modulator. With these arrangements, the related arts provide a rectangular image from a projection lens.